High-density carbon-carbon composites are extremely strong materials capable of withstanding high temperatures. Such composites find use in the manufacture of structural parts for high-performance use, high-performance coatings, semi-conductor encapsulators and high-performance insulators. Examples of structural parts made from these materials are as heat shields for reentry vehicles and solid propellant rocket motor nozzles.
These composites are formed from resins, phenolic resins being in general use at present, by pyrolysis. Certain qualities of the resins are significant in terms of their processing and performance as well as the characteristics of the final product. These resin qualities include solubility, flow characteristics, and char yield upon pyrolysis of the resin, as well as mechanical properties of the final product. A high char yield, or low volatiles content, is particularly important, since it relates to the minimization of weight loss shrinkage, pores and cracking upon graphitization. The char yield may be expressed as the weight remaining after thermal treatment to 1000.degree. C.
The phenolic resins in current use generally have char yields of less than 50%, due to the release of such decomposition products as water, carbon monoxide, phenol and methane upon pyrolysis and carbonization. With such a high quantity of volatiles produced, the resulting composite is porous, low in density and susceptible to stress due to matrix shrinkage. To compensate for these deficiencies, the composite after first having been formed is impregnated with coal tar pitch and repyrolysed (i.e., "densified"). Five to seven densification cycles are generally required to achieve a product with thermostructural properties adequate for high performance use.